The effects of oxidizing conditions on vulcanizates obtained from polymers having carbon-carbon double bond unsaturation have long been a problem, particularly in applications where the vulcanizates are exposed to elevated temperatures for extended periods of time. A variety of approaches have been developed in the art in an attempt to solve this problem.
It is known that the carbon-carbon double bonds of such polymers activate the vulcanizate to oxidative attack. One solution to the problem of oxidative attack is to use polymers with few or no carbon-carbon double bonds. Examples of such polymers include butyl rubber (copolymers of isobutylene and isoprene) which typically contain from about 0.5 to about 3.0 mole percent of carbon-carbon double bond unsaturation, and ethylene-propylene copolymers which contain no such unsaturation.
Certain applications, such as the various hoses and seals in the engine compartment of automobiles, require vulcanized polymers with a combination of oil resistance, and resistance to oxidative attack in air at elevated temperatures for extended periods of time. Vulcanizates of copolymers of conjugated dienes and α,β-unsaturated nitrites, such as acrylonitrile-butadiene copolymer, commonly known as nitrile rubber or NBR, are well known for their oil resistance. However, they contain carbon-carbon double bond unsaturation and therefore are susceptible to oxidative attack unless subjected to special compounding procedures for the production of oxidation resistant vulcanizates.
In order to reduce the amount of carbon-carbon double bond unsaturation in NBR and yet retain the copolymer's oil resistance which is thought to be provided by the nitrile functional groups in the copolymer, methods have been developed to selectively hydrogenate the carbon-carbon double bond unsaturation of NBR without hydrogenating the nitrile groups to produce hydrogenated NBR or HNBR. Such hydrogenation is e.g. disclosed in British patent 1,558,491, the contents of which are hereby incorporated by reference. A review by Tan et al. Rubber Chem. Tech. (2005), 78(3), 489-515 gives a comprehensive overview in this technical field.
While the development of HNBR has been a significant advance in the art, there is still room for improvement.
Campomizzi et al. (U.S. Pat. No. 6,214,922) have successfully demonstrated that the usage of a combination of antioxidants with bases significantly improves the hot air aging characteristics of hydrogenated nitrile vulcanizates. One preferred combination includes the usage sodium carbonate which has given the best results so far.
Other publications in the field are e.g. Arnoldi et al. (US 2002/072557 A1), wherein anti-aging agents for organic polymers based on salts of sterically hindered phenolic compounds with at least two phenolic OH groups are disclosed.
However, even though the vulcanizates mentioned in U.S. Pat. No. 6,214,92 (Campomizzi) are very useful for high temperature applications, when the vulcanizates mentioned in U.S. Pat. No. 6,214,92 are in contact with aqueous media, especially water, high swelling is observed limiting the usage of sodium carbonate for high temperature applications in the presence of aqueous media or water vapor. Specifically, there is a need to develop polymer vulcanizates with improved physical properties such as hot air aging having a swelling resistance to polar media such as water and aqueous systems.
It is an object of the present invention to obviate or mitigate at least one of the above-mentioned disadvantages of the prior art.
It is another object of the present invention to provide a novel polymer vulcanizate.
It is yet another object of the present invention to provide a novel process for producing said novel polymer vulcanizate.
It is yet another object of the present invention to provide a novel polymer composition for producing a polymer vulcanizate.
It is yet another object of the present invention to provide a novel method for improving the hot air aging characteristics of a polymer vulcanizate and at the same time improving the swelling resistance to polar media.